Conventional surface mount resistors have wrap-around terminals on the ends of the resistor. When such surface mount resistors are soldered to a printed circuit board, solder covers entire surface of the terminals forming a fillets, resulting in occupation of an additional area for mounting. One example of such a conventional surface mount resistor is found in EPO 0810614A1 to Hashimoto et al. A flip chip resistor is a resistor that has no side electrodes and is soldered with its printed side towards the printed circuit board. With this configuration, the solder fillets are not formed thus decreasing the amount of circuit board space required and increasing the mounting density particularly in the case of small chip sizes.
Two examples of prior art flip chip resistors are shown in FIGS. 1 and 2. The flip chip resistor shown in FIG. 1 is described in U.S. Pat. No. 6,023,217 to Yamada et al. The flip chip resistor of FIG. 1 improves the quality of mounting and insulation between the printed layers of the resistor and a printed circuit board which is important when there is a printed circuit board trace running between the terminations.
A second prior art attempt at a flip chip resistor is shown in FIG. 2. The device shown in FIG. 2 has been offered by a number of chip manufacturers.
Both of these prior art flip chip resistors have problems. In particular, the area of conductive layers disposed under the joint of a protective overcoat layer and plated Nickel barrier disposed over a Silver electrode is subjected to destructive influence of environmental conditions more than other inner parts of the flip chip resistor because this joint is usually not sufficiently hermetic. This results in reduced reliability, especially in cases of face down mounting when residual flux cannot be reliably removed from the overcoat surface. Therefore, these flip chip resistors require expensive conductive materials based on noble metals (i.e. Pd, Au, Pt) for the top conductive layers in order to prevent erosion of the conductive layers.
A further problem with these configurations is that the pads provided are too small for reliable soldering. This problem becomes even more important in the case of small chip sizes. The pad areas in these prior art designs can only be enlarged when the resistance layer size is changed. Such a change interferes with requirements for laser trimming. Therefore, problems in the art remain.
Thus, it is a primary object of the present invention to improve upon the state of the art.
Another object of the present invention is to provide a flip chip resistor with high reliability.
Yet another object of the present invention is to provide a flip chip resistor that can be manufactured at a low cost.
As a further object of the present invention to provide a flip chip resistor that can be manufactured in small chip sizes.
A further object of the present invention is to provide a flip chip resistor that allows for sufficiently large pads for reliable soldering even when the flip chip resistor is of small size.
These and other objects, features and advantages of the present invention will become apparent from the description and claims that follow.
The present invention relates to a flip chip resistor.
According to one aspect of the invention, the flip chip resistor includes a substrate having opposite ends, a pair of electrodes, formed from a first electrode layer disposed on the opposite ends of the substrate, a resistance layer electrically connecting the pair of electrodes, a protective layer overlaying the resistance layer, and a second electrode layer overlaying the first electrode layer and at least a portion of the protective layer and optionally a portion of the resistance layer. A plating layer can then be overlayed on the second electrode layer to provide for solder attachment to a printed circuit board. This allows the flip chip resistor to be surface mounted with the resistance layer positioned towards the printed circuit board and results in high reliability.
According to another aspect of the present invention, a method of manufacturing flip chip resistors is provided. The method includes applying a first electrode layer to a substrate to create pairs of opposite electrodes, applying a resistance layer between each pair of opposite electrodes, applying a first protective layer at least partially overlaying the resistance layer, applying a second protective layer at least partially overlaying at least a portion of the resistance layer, and applying a second electrode layer overlaying the first electrode layer and at least a portion of the second protective layer. The substrate can then be divided to form individual flip chip resistors.
The present invention provides for an array of resistors to be manufactured using the above method. In a resistor chip array, multiple flip chip resistors are disposed on the same substrate.
The configuration of the present invention increases reliability of flip chip resistors, does not require expensive conductive materials for the electrode layers, and is especially advantageous in the case of small chip sizes as pad areas or electrode areas are large enough to promote reliable soldering.